The invention relates to elements faced with superhard material, and particularly to preform elements comprising a facing table of superhard material having a front face, a peripheral surface, and a rear surface bonded to a substrate of material which is less hard than the superhard material.
Preform elements of this kind are often used as cutting elements on rotary drag-type drill bits, and the present invention will be particularly described in relation to such use. However, the invention is not restricted to cutting elements for this particular use, and may relate to preform elements for other purposes. For example, elements faced with superhard material, of the kind referred to, may also be employed in workpiece-shaping tools, high pressure nozzles, wire-drawing dies, bearings and other parts subject to sliding wear, as well as elements subject to percussive loads as may be the case in tappets, cams, cam followers, and similar devices in which a surface of high wear resistance is required.
Preform elements used as cutting elements in rotary drill bits usually have a facing table of polycrystalline diamond, although other superhard materials are available, such as cubic boron nitride. The substrate of less hard material is often formed from cemented tungsten carbide, and the facing table and substrate are bonded together during formation of the element in a high pressure, high temperature forming press. This forming process is well known and will not be described in detail.
Each preform cutting clement may be mounted on a carrier in the form of a generally cylindrical stud or post received in a socket in the body of the drill bit. The carrier is often formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carrier, for example by a process known as "LS bonding". Alternatively, the substrate itself may be of sufficient thickness as to provide, in effect, a cylindrical stud which is sufficiently long to be directly received in a socket in the bit body, without being brazed to a carrier. The bit body itself may be machined from metal, usually steel, or may be moulded using a powder metallurgy process.
Such cutting elements are subjected to extremes of temperature during formation and mounting on the bit body, and are also subjected to high temperatures and heavy loads when the drill is in use down a borehole. It is found that as a result of such conditions spalling and delamination of the superhard facing table can occur, that is to say the separation and loss of the diamond or other superhard material over the cutting surface of the table.
This may also occur in preform elements used for other purposes, and particularly where the elements are subjected to repetitive percussive loads, as in tappets and cam mechanisms.
Commonly, in preform elements of the above type the interface between the superhard table and the substrate has usually been flat and planar. However, particularly in cutting elements for drill bits, attempts have been made to improve the bond between the superhard facing table and the substrate by configuring the rear face of the facing table so as to provide a degree of mechanical interlocking between the facing table and substrate.
One such arrangement is shown in U.S. Pat. No. 512,327 where the rear surface of the facing table is integrally formed with a plurality of identical spaced apart parallel ribs of constant depth. The facing table also includes a peripheral ring of greater thickness, the extremities of the parallel ribs intersecting the surrounding ring. U.S. Pat. No. 4,784,023 illustrates a similar arrangement but without the peripheral ring.
In most prior art arrangements the ribs generally all project into the substrate to substantially the same depth. In cases where the ribs project into the substrate for different depths, the ribs of different depths are arranged in a regular pattern which is commonly symmetrical about some axis of the preform element.
Thus, in the prior art arrangements the rear extremities of the ribs, as they project into the substrate, lie on an imaginary surface which is substantially smooth and continuous. In the case where the ribs are of constant depth, such imaginary surface will be a flat plane.
As a result, the provision of such ribs may do little or nothing to inhibit the propagation of cracks through the substrate. Thus, a crack may be initiated at the rearward extremity of a single rib, and if the ribs are of the same depth, or if their extremities otherwise lie on a smooth plane, there is nothing to prevent the crack from propagating completely across the rearward extremities of all the ribs. The present invention provides an improved design of preform element where such crack propagation may be more effectively inhibited.